Schumann resonances

Earth's surface and the lower ionosphere form a natural waveguide for extremely low-frequency electromagnetic waves. Lightning discharges excite waves that can travel around the planet; waves with suitable wavelengths reinforce one another, producing resonances. The resonances are named for physicist Winfried Otto Schumann, who published theoretical work predicting them in 1952. Reliable measurements followed in the early 1960s.

The resonances form a series of spectral peaks rather than a single fixed tone. Commonly reported peaks lie near 8, 14, 20 and 26 hertz. The values are approximate: the Earth–ionosphere system is not an ideal, perfectly conducting cavity, and its electrical properties and geometry vary. Observations and models therefore account for peak widths and changes in frequency and intensity.

Lightning is the principal natural source that excites the global resonances. Because lightning activity varies in location and time, measurements of resonance signals can help researchers investigate global lightning activity and related atmospheric electrical conditions. NASA describes the resonances as useful for analyzing Earth's weather and electrical environment. Interpretation requires care: a change in a measured peak is not, by itself, evidence of a particular cause or an effect on human health.